1. Heat oven to 350 degree F. Coat two 15 x 10-inch jelly-roll pans with nonstick cooking spray. Pour lemon juice into 4-cup measuring cup; add enough water to equal 2-2/3 cups.
2. In large bowl, beat cake mixes, egg whites, lemon zest, juice-water mixture and lemon extract to blend, about 3 minutes. Divide evenly between two prepared pans; smooth tops.
3. Bake at 350 degree F until cake springs back when lightly touched, about 20 minutes. Let cool on wire racks 10 minutes. Place rack over each cake. Carefully invert cakes and racks. Carefully remove pans from cakes and let cool to room temperature. If making ahead, wrap layers when cool in plastic wrap; refrigerate for several days. Bring to room temperature before proceeding.Frosting:
4. Beat butter and shortening in large bowl 1 to 2 minutes until fluffy. Add confectioners sugar, milk and lemon juice. Continue beating on low until well blended and good spreading consistency, adding more milk if needed.Decoration:
5. Place one layer on serving tray. Reserve 1 cup frosting for piping. Frost layer with 1 cup frosting; top with second layer; frost top and sides. Place reserved frosting in pastry bag with a star tip. Outline cake with piped stars.
6. Mark off a field for stars with toothpicks6 inches across and 4-1/2 inches down. Fill in with blueberries.
7. Place seven red stripes of raspberries on cake as shown, leaving equal spacing for 6 white stripes between them. Carefully pipe white frosting stripes between the rows of raspberries. Refrigerate uncovered to set frosting, then loosely cover with nonstick aluminum foil. Refrigerate until ready to serve. Let stand at room temp for 20 minutes before serving. Makes 35 servings.